We
currently live in an icehouse  a climate in which large continental
ice sheets exist, in this case at both poles. The onset of this icehouse began
in Antarctica 34 million years ago and in the Arctic about 2 million years ago.
The latter stages of human evolution occurred in this bipolar icehouse, and
human civilization unfolded during the relatively stable, most recent interglacial
phase of this icehouse (the glacial times are popularly known as ice ages).
This particular climate state, however, represents only a fraction of 1 percent
of Earths history. Thus, humans evolved during, and are adapted to, an
atypical climate state.

Geologists hike to the base of the retreating
Athabasca Glacier in Banff National Park in Alberta, Canada. Modern glaciers
are part of the current icehouse, with ice present at both poles.
Courtesy of Lynn Soreghan.

But although we technically live in an icehouse, we may be embarking on a one-way
voyage into a permanently deglaciated greenhouse. To grasp what may be in store
for the planet, we can look at the deep-time geologic record, which
archives a nearly-billion-year-long record of several icehouses. Each of these
icehouse periods was associated with abrupt climate change over a range of timescales,
and each provides insight into the climatically fickle transitions from icehouse
to greenhouse states.

Earths Icy Times

To understand our planets present and anticipated future climate, paleoclimatologists
have typically turned to the recent history of our current icehouse  roughly
the last two million years, or the Quaternary, and particularly the last 10,000
to 100,000 years. More than a decade of research evaluating ancient atmospheres
entombed in ice, as well as inferring past surface temperatures from the chemical
composition of ice cores and ancient marine organisms, has documented the potential
volatility of our global climate system.

These studies, which provide information at a resolution of decades to millennia
 well within human comprehension  also reveal that prior to the
last century, Quaternary atmospheric carbon dioxide levels never surpassed 280
parts per million. These carbon dioxide levels are considerably lower than our
present-day atmospheric content of 379 parts per million, which continues its
perilous rise at the rate of 3 parts per million per year. In a 2002 Nature
paper, Lee Kump of Pennsylvania State University suggested that carbon dioxide
concentrations in the atmosphere could surpass 2,000 parts per million by the
time our current estimated fossil fuel resources are exhausted. Furthermore,
the spectrum of documented climate variation over the past half-million years
is mild relative to the anticipated average warming on Earth of 5 to 12 degrees
Celsius, under such high levels of greenhouse gases.

Notably, the magnitude of the climate change we are currently witnessing has
no precedent in human history and no precedent in the history of our modern
icehouse. Rather, projected atmospheric carbon dioxide levels for the end of
the 21st century were last seen on Earth more than 30 million years ago, coincident
with the transition into our present icehouse state.

Thus to find a possible analog for modern-day conditions, it is necessary to
dig back deeper in time; Earth has experienced icehouse climates far more extreme
than those of the past 2 million years. Despite distinct differences between
each icehouse period, several aspects of Earths climate and its link to
the living world are common during icy times. Geologic records of transitions
into and out of past icehouses contain the characteristic fingerprint of change
caused by shifts in greenhouse gases, at times accompanied by increases up to
several fold in atmospheric carbon dioxide levels.

Rapid climate shifts were the norm during these major transitions (icehouse-to-greenhouse
and vice versa), including significant change in surface temperatures and precipitation
patterns across the globe. Notably, recent studies of the major shift into our
current icehouse and of the icehouse-greenhouse transition about 300 million
years ago suggest that Earth might be increasingly primed for abrupt
climate change when initial background carbon dioxide levels are low. Understanding
these transitions in the deep-time climate record is thus pivotal to understanding
potential future abrupt change.

Finding the evidence

Generally speaking, icehouse intervals are rare in Earths history. The
fundamental evidence for past glaciation is found in the field either as scars
in the landscape where ice eroded the surface, or locked in sedimentary deposits
directly or remotely affected by ice. Exhumed ancient glacial valleys dating
from more than 300 million years ago have been discovered on at least a few
continents, and fossil ice streams as old as 440 million years have been documented
in Africa. Continental-scale ice masses have left behind a range of physical
evidence, from preservation of ice-rafted debris to surfaces scratched by debris-infused
ice.

Sediment deposited at sites that were once located at low latitudes, far removed
from ice sheets, provide perhaps the most complete and sensitive record of glacial-interglacial
climate changes. These include units known as cyclothems, composed of vertically
stacked layers of marine and continental deposits that formed in environments
not typically juxtaposed. Cyclothems, together with repeating couplets of fossil
soils and wind-blown deposits, track the modulating effects of ice sheets on
sea level and climate. Many of these sedimentary packages crop out on modern
mountain and canyon exposures, while others are best retrieved through drilling.

Ancient ice also left chemical fingerprints of climate change locked in sedimentary
rocks and in the shells of fossil organisms contained within them. The major
and minor elements in sedimentary deposits are proxies of weathering by surface
processes and can be used to distinguish various climates.

Changes in the composition of the oceans brought on by the growth and melting
of continental ice sheets, along with temperature variations, are recorded in
the oxygen isotopic composition of the fossil shells. Similarly, changes in
terrestrial and marine biomass are captured by the carbon isotopic composition
of these ancient shells. Ancient atmospheric composition and temperature are
imprinted in minerals that grew in fossil soils.

Another independent and complementary record of past environmental change can
be found in the often exquisitely preserved fossil remains of ancient animals
and plants. Field, laboratory and statistical studies of marine and terrestrial
fossils, including fossil roots and pollen, provide insight into the distribution
of organisms, their response to environmental changes and their evolutionary
patterns. Some fossils preserve soft body parts, including cellular structure
and individual molecular compounds, that provide an added dimension to unraveling
Earths paleoclimate history.

A deep-freeze plunge

Deposits that are 580 million to 750 million years old that are currently exposed
on land on multiple continents hold field and geochemical evidence for multiple
transformations of Earth between super-greenhouse and snowball-icehouse
states during the Neoproterozoic. The last of these extreme climatic excursions
may have provided the trigger that led to the evolution of multicellular animal
life.

At least three times during this period of Earths history, the planet
dipped into a deep freeze, during which time ice sheets penetrated
deep into the tropics. The extent to which all oceans were frozen over remains
debated, with some researchers suggesting that the snowball Earth,
at times, may have been more of a slushball (see Geotimes, December
2005).

Buildup of atmospheric carbon dioxide  possibly from levels lower than
present-day values during glacial periods to levels up to 350 times higher 
de-iced the globe. Once the climate switch flipped, Earth veered from frigid
snowball to steamy hothouse (possibly with average surface temperatures of 50
to 60 degrees Celsius) within centuries to millennia. The removal of excess
carbon from the ocean-atmosphere system would have provided the brake
on runaway greenhouse conditions and pushed the planet into its subsequent ice
age, but not until geologically slow processes such as mineral weathering on
land and burial of carbon in the ocean reestablished equilibrium.

The Neoproterozoic ice ages were anomalous given the overall warm climate of
the Precambrian world and serve to illustrate the planets full potential
for climate variability, especially when prodded by changes in atmospheric carbon
dioxide. The extreme nature of the yo-yo-like climate at this time reflects
the unusual distribution of supercontinents around the equator, our suns
heat engine that was idling at two-thirds of its current level, and the fantastically
high reflectivity of an ice-covered snowball Earth.

Ultimately, however, these ice ages, which were more extreme than any ever again
experienced on Earth, may literally reflect out-of-this-world conditions. Last
year, Alexander Pavlov of the University of Colorado in Boulder and colleagues
suggested that the planets deep chill may have occurred as the result
of its passage through dense cosmic megaclouds filled with interstellar dust
that would have limited the penetration of the suns heat energy to Earth.

In the end, the environmental stresses, which nearly snuffed out the single-celled
life that lived on the Neoproterozoic Earth with each freeze-fry
cycle, may have been the requisite forcing factor for the evolution and radiation
of multicellular animal life. This shift, however, was not without major environmental
consequences. The ramifications of such environmental perturbation included
acidified oceans during greenhouse intervals, recurrent major reorganization
of the global carbon budget, and periods of sustained extraordinary surface
winds and ocean waves.

The take-home lesson is that Earth is capable of accommodating major perturbation
to global carbon cycling, including significantly elevated greenhouse gas contents
in the atmosphere, but often at timescales that exceed the time needed for life
to recover.

Life in the Icehouse

After the Cambrian Explosion 545 million years ago, the Late Ordovician
glaciation (about 440 million years ago) was the first of three icehouse intervals
that affected Earth once it was populated by animal life. From onset to meltdown,
this glacial cycle was complete in less than 1 million years, but left in its
wake Earths second largest extinction.

The Ordovician ice age, like the current icehouse, initiatiated a polar ice
cap in the southern hemisphere, and had glaciation and sea-level cycles driven
by the waxing and waning of continental ice sheets in tune with orbital rhythms.
Its initiation under an atmosphere with carbon dioxide contents roughly eight
to 12 times higher than present-day levels, however, is unique.

Continental-scale glaciation under such greenhouse conditions attests to the
complex climate dynamics and the susceptibility of the climate system to factors
beyond atmospheric composition. Like all of Earths other icehouses, the
destiny of this climatic interval was dictated by threshold levels of carbon
dioxide and a point of no (ice) return.

Life on Earth at the time, which was limited beyond the marine realm to liverworts
on land, had radiated and diversified under greenhouse climates, and thus may
have been particularly extinction-prone to the geologically sudden onset of
glaciation. It is not surprising that both the transition into and out of this
ice age are linked to biotic crises that together witnessed the loss of 85 percent
of all species on Earth.

Hints of a world to come

Some 100 million years after the Ordovician, Earth experienced its longest and
most widespread glaciation, referred to as the Late Paleozoic Gondwanan Ice
Age. Between 330 million and 270 million years ago, the serendipity of global
mountain-building processes and associated basin formation, occurring while
the supercontinent was assembling, enabled exquisite and widespread preservation
of rock deposits worldwide that have been the subject of decades of research.
Significantly, the Late Paleozoic Gondwanan Ice Age is the only example of climate
change in an icehouse on a vegetated Earth, making it the nearest analog to
Earths current state  and the only example of an icehouse-greenhouse
transition that records the impact on animal and plant ecosystems in both the
marine and terrestrial realms.

Similar to the current icehouse, continental ice sheets first developed in the
southern hemisphere under generally low atmospheric carbon dioxide concentrations,
and arguably may have eventually affected both hemispheres. Although the Late
Paleozoic interval is considered Earths best-understood pre-Quaternary
icehouse, emerging research reveals a more complex climate history than conventionally
thought. Rather than a single, uninterrupted icehouse characterized by widespread
ice in the southern hemisphere, this icehouse might have archived a series of
shorter-lived glaciations interrupted by warmer intervals. Conversely, emerging
evidence from equatorial regions hints at the possibility of surprisingly cold
temperatures periodically, implying potentially severe climatic conditions in
the very sensitive tropics.

Cyclothems have long marked the Gondwanan icehouse as a time of relatively rapid
and high-magnitude fluctuations in sea level as well. Only recently have researchers
come to link these sea-level changes to climate change  resurrecting an
idea first proposed 70 years ago. The huge shifts in shorelines recorded in
the cyclothems reflect large-magnitude water transfers  as water previously
locked in ice melted out and returned to the oceans, and cycled back again.

The ultimate climatic drivers of these glacial-interglacial shifts remain nebulous.
As we probe the record at finer and finer scales, however, it becomes clear
that this toggling had significant and wide-ranging repercussions.

Large-scale shifts in atmospheric circulation patterns in low latitudes, for
example, resulted in significant changes in relative humidity, temperature,
wind strength and direction, and even atmospheric dust loads; the thickest windblown
dust deposits yet recognized in the geologic record occurred during the Gondwanan
icehouse. The significant shifts in Earths climate triggered large-scale
reorganizations of the marine and terrestrial biosphere, as repeatedly changing
habitats precipitated widespread migration, evolution, extinction and diversification
in many groups, particularly in the tropics.

The ultimate demise of this Gondwanan Ice Age was the last time in Earths
history to witness a transition to an ice-sheet-free world, a condition that
lasted until 34 million years ago. The climate history that is unfolding for
this paleo-deglaciation mirrors the newly emerging record of atmospheric composition
and climate instability that foreshadowed the transition into our present glacial
state. This implies that rapid fluctuations in atmospheric carbon dioxide and
surface temperatures may be the climatic norm during Earths transitions
from one major state to another. Anticipated anthropogenically driven climate
change may ultimately lead to such a climate state turnover, but at a rate unique
to the history of this planet.

A few years ago, Sir John Houghton, former co-chair of the Intergovernmental
Panel on Climate Changes working group, described modern global warming
as a weapon of mass destruction. If modern global change heralds
the exit from our current icehouse, then the history of icehouse terminations
and biospheric repercussions as archived in the geologic record suggests this
is indeed an apt description.

Human civilization has developed during an extremely rare climate, geologically
speaking, one with large-scale ice sheets at both poles. Only recently
have we begun to understand that what we think of as normal
climate is not in fact normal (see main story). Earths
climate changes perpetually, far beyond the limits known from the modern
and near-modern worlds.

That Earths climate has lurched to extremes far beyond human experience
consoles some people  why worry if extremes lie within Earths
comfort zone of natural variation? But with our carbon dioxide output
rising at an unprecedented rate, we are now exiting the comfort zone of
human experience, following an accelerated synthetic path. The planet
is warming perceptibly, and globally averaged temperature projections
forecast an increase of several degrees Celsius within the century. The
outcome of forcing the planet into a greenhouse during what
is otherwise an icehouse is currently beyond prediction.

Model predictions of climate states for the near future (next 20 to 200
years) exhibit considerable variability, which is inherent to the uncertainty
of the science. Scientists must improve knowledge and their presentation
of that knowledge to help decision-makers prepare for the future and manage
for the consequences of the human presence on this planet. But we cannot
claim to be maximizing our contribution to this discussion when 99.96
percent of Earths climate behavior remains virtually unexplored.

While Earths near-time geologic past teems with high-resolution
climate data, and studies of the last 2 million years have refined our
understanding of our nearest icehouse relative, they archive only a tiny
slice of Earths history. It is necessary to find and understand
more analogs for our near future with high levels of carbon dioxide. Ice-core
data demonstrate we are experiencing the highest levels of atmospheric
carbon dioxide ever breathed by our species; even with drastic emissions
reductions, our grandchildren will experience values known only from Earths
deep-time past.

Powerful developments in our abilities to read, date and model Earths
history are elevating the reach and relevance of deep-time climate research.
A decade ago, Earths climate was not imagined to have veered to
such extremes as are now recognized for the geologic past, nor at such
rates, except within the realms of planetary geology or science fiction.
Our thinking had been overly conditioned by Earths modern and recent
states.

Still, uncertainty in assessing future climate remains because key components
of the system have not been fully, or even mostly, investigated. We need
a holistic understanding of the system  an understanding that can
be developed only by exploring the full spectrum of events that have occurred
on Earth. The need for this holistic understanding of the deep past is
driving the grassroots GeoSystems effort (www.geosystems.org), which is
a multidisciplinary community of researchers formed to raise awareness
of existing and potentially new high-resolution data mined from deep time,
to better understand Earths climate history.

Compiling such information helps scientists to address key unknowns of
climate-system behavior. Processes controlling abrupt change, the effects
of aerosols, the identity and role of feedbacks, and the effects of alternative
oceanic circulation, for example, are reachable in deep time. On long
geologic timescales, the biosphere and atmosphere have clearly co-evolved,
and the paleobiologic record houses the history of lifes response
and recovery to alternative Earth climate states. Additionally, interactions
between climate and tectonics remained below the radar of our collective
consciousness until only recently, but are clearly key, and uniquely suited
for deep-time study.

Researchers in a variety of fields are coming together to better understand
deep-time paleoclimate, using a robust geoinformatics system to host data
and enable analysis, synthesis and correlation. Integrating these advances
in data with developments in climate modeling offers the potential to
exploit deep-time datasets to improve prediction with reduced uncertainty.

Climate research in deep time adds a new dimension to the climate change
discussion. Policy-makers must understand that few final answers
exist in science, and that we must continually push the envelope of our
knowledge to ensure that their decisions are the best possible. Climate
research in deep time and near time are not adversarial, but symbiotic
 each contributing needed dimensions to the whole.

Significant discoveries await our concerted efforts, and our open minds.
Put simply, if we wish to predict the near future, we need to probe the
deep past.

Soreghan is an associate
professor in the School of Geology and Geophysics at the University of
Oklahoma. Snyder is a professor in the Department of Geosciences at Boise
State University. Both are involved in the GeoSystems effort (www.geosystems.org).

Montañez is a professor
in the Department of Geology at the University of California in Davis. Soreghan
is an associate professor in the School of Geology and Geophysics at the University
of Oklahoma. Both authors are involved in the nascent GeoSystems effort (see sidebar,
above).